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Process Management
A process is a program in execution. It is a unit of work within the system.
Program is a passive entity, process is an active entity.
Process needs resources to accomplish its task
– CPU, memory, I/O, files
– Initialization data
Process termination requires reclaim of any reusable resources
Single-threaded process has one program counter specifying location
of next instruction to execute
– Process executes instructions sequentially, one at a time, until
Multi-threaded process has one program counter per thread
Typically system has many processes, some user, some operating system
running concurrently on one or more CPUs
– Concurrency by multiplexing the CPUs among the processes / threads
Process Management Activities
The operating system is responsible for the following
activities in connection with process
• Creating and deleting both user and system
• Suspending and resuming processes
• Providing mechanisms for process
• Providing mechanisms for process communication
• Providing mechanisms for deadlock handling
Memory Management
• All data in memory before and after processing
• All instructions in memory in order to execute
• Memory management determines what is in
memory when
– Optimizing CPU utilization and computer response to
• Memory management activities
– Keeping track of which parts of memory are currently
being used and by whom
– Deciding which processes (or parts thereof) and data
to move into and out of memory
– Allocating and deallocating memory space as needed
Storage Management
• OS provides uniform, logical view of information storage
– Abstracts physical properties to logical storage unit - file
– Each medium is controlled by device (i.e., disk drive, tape
• Varying properties include access speed, capacity, data-transfer
rate, access method (sequential or random)
• File-System management
– Files usually organized into directories
– Access control on most systems to determine who can
access what
– OS activities include
Creating and deleting files and directories
Primitives to manipulate files and dirs
Mapping files onto secondary storage
Backup files onto stable (non-volatile) storage media
Mass-Storage Management
Usually disks used to store data that does not fit in main memory or data that
must be kept for a “long” period of time
Proper management is of central importance
Entire speed of computer operation hinges on disk subsystem and its algorithms
OS activities
– Free-space management
– Storage allocation
– Disk scheduling
Some storage need not be fast
– Tertiary storage includes optical storage, magnetic tape
– Still must be managed
– Varies between WORM (write-once, read-many-times) and RW (read-write)
Migration of Integer A from Disk to Register
• Multitasking environments must be careful to use most recent
value, no matter where it is stored in the storage hierarchy
• Multiprocessor environment must provide cache coherency in
hardware such that all CPUs have the most recent value in
their cache
• Distributed environment situation even more complex
– Several copies of a datum can exist
– Various solutions covered in Chapter 17
I/O Subsystem
• One purpose of OS is to hide peculiarities of
hardware devices from the user
• I/O subsystem responsible for
– Memory management of I/O including buffering
(storing data temporarily while it is being
transferred), caching (storing parts of data in
faster storage for performance), spooling (the
overlapping of output of one job with input of
other jobs)
– General device-driver interface
– Drivers for specific hardware devices
Protection and Security
• Protection – any mechanism for controlling access of
processes or users to resources defined by the OS
• Security – defense of the system against internal and
external attacks
– Huge range, including denial-of-service, worms, viruses,
identity theft, theft of service
• Systems generally first distinguish among users, to
determine who can do what
– User identities (user IDs, security IDs) include name and
associated number, one per user
– User ID then associated with all files, processes of that user
to determine access control
– Group identifier (group ID) allows set of users to be defined
and controls managed, then also associated with each
process, file
– Privilege escalation allows user to change to effective ID
with more rights